Shoe soles require a shock absorbing function of absorbing and cushioning shocks of landing in addition to lightness in weight and a function of stably supporting the foot. Recently, some shoes have been proposed, which includes a restitution function as well as above functions. The restitution function is a function wherein the shock energy at the time of landing is stored in a shoe sole as deformation energy and the stored deformation energy is released at the time of leaving the ground. This function serves to enhance athletic ability of a wearer.
The deformation energy is stored in an element of the shoe sole by compression, bending or the like of the element. However, in general, viscoelastic materials having small Young's modulus such as resin foam, which is commonly used as a cushioning member of shoe soles, cannot exhibit high restitution function because energy is dissipated as heat etc. at the time of deformation.
The following documents disclose shoe sole structures having the above restitution function.    The first patent document: Japanese Patent Laid-Open No. 10-257904 (abstract)    The second patent document: Japanese Patent Laid-Open No. 10-262706 (abstract)    The third patent document: Japanese Patent Laid-Open No. 03-026202 (abstract)    The fourth patent document: Japanese Patent Laid-Open No. 01-274705 (abstract)    The fifth patent document: U.S. Pat. No. 6,598,320 (abstract)    The sixth patent document: U.S. Pat. No. 6,694,642 (abstract)    The seventh patent document: U.S. Pat. No. 6,568,102 (abstract)
In the shoes disclosed in Japanese Patent Laid-Open No. 10-257904 and Japanese Patent Laid-Open No. 10-262706, the entire rear foot portion is supported by a single plate spring. Thus, the support for the rear foot portion may become unstable.
In the shoes disclosed in Japanese Patent Laid-Open No. 10-257904, Japanese Patent Laid-Open No. 10-262706 and Japanese Patent Laid-Open No. 03-026202, a plate spring or a coil spring is placed in the rear foot portion. Thus, since energy is stored in these springs, high restitution function may be obtained. However, these shoes may be hard to dissipate the shock and be unable to support the foot stably.
The spring 101, disclosed in Japanese Patent Laid-Open No. 03-026202, is housed within the shoe sole. In such a structure, a part of impact energy at the time of landing is absorbed and dissipated in the sole, and the remainder of the energy is absorbed by the spring. Accordingly, the amount of energy that can be stored by the spring is reduced.
The shoe disclosed in Japanese Patent Laid-Open No. 01-274705 has a cavity which is formed in the shoe sole. A reaction plate is built in this cavity. The reaction plate has upper and lower facing sides and fore and rear curved parts that connect the upper and lower facing sides. A gel cushioning member is provided in the reaction plate.
Since the reaction plate housed in the shoe sole also in this prior art shoe, the shoe has similar demerits to the shoe of Japanese Patent Laid-Open No. 03-026202. It is supposed that the part in which deformation energy is stored due to shock of landing, is mainly the fore and rear curved parts, not the upper and lower facing sides.
In this shoe, impact load (shock force) of landing is applied to the oval spring after having been dispersed in the sole. Accordingly, since the dispersed impact load is applied on each part of the oval spring as distributed load, the amount of deflection of the endless spring is considered to be small. Therefore, impact energy cannot be stored in the oval spring sufficiently.
U.S. Pat. No. 6,598,320 and U.S. Pat. No. 6,694,642 fail to disclose a bending deformation member being generally V-shaped or U-shaped in section.
In the shoe sole disclosed in U.S. Pat. No. 6,658,102, the plate is arranged to extend over the whole of the rear foot part.